Solid state controlled rectifier circuits and apparatus



Sept. 22, 1964 SOLID STATE H. C. KAEDING Filed Sept. 17, 1962 l SCRl 2l "s J u F l D 6 1 INVENTQR.

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United States Patent Office 3,lll,3ll7 Patented Sept. 22, 1964 3,156,367 SGLID STATE CONTROLLED RECTEFER CIRCUITS AND APPARATUS li-Iugo C. Kaeding, Fort Wayne, ind., assigner to General Electric Company, a corporation of New York Filed Sept. 17, i962, Ser. No. 224,114 1 Claim. (till. S18-345) This invention relates to solid state controlled rectifier circuits yand apparatus. More particularly, it relates to such circuits and apparatus for controlling the average power to a load, such as a motor, operated from a direct current source.

The power supplied to a load operated from a battery or a filtered rectified alternating source may be selectively varied by controlling the average power applied to the motor with solid state power devices, which are switched on at selected frequencies to provide controlled pulses of -current to the load. Where a solid state controlled rectifier is 4used as a power device in conjunction with a battery to supply power, it is not only necessary to switch the rectifier into a conducting state but also to commutate the controlled rectifier.

After a signal is applied at the gate of a silicon controlled rectifier, it will conduct in a forward direction and continue to conduct after the gate signal is removed. In order that the firing circuit may regain control of the controlled rectifier, it is necessary to reduce the anode voltage to Zero or to apply ya reverse bias for a short interval of time in order to turn off the controlled rectifier. When silicon lcontrolled rectifiers are used in conjunction with cyclical power supplies, such as an unfiltered rectified power source or an alternating power source, the reduction of the voltage to Zero at the end of each half cycle or the reversal of the voltage of an alternating source may be used to turn off or commutate the controlled rectifier and thereby permit the firing circuit to regain control of the power device.

Heretofore, difficulties have been encountered in designing control circuits for selectively controlling loads such as DC. motors operated from a battery or a filtered alternating supply, where silicon controlled rectifiers are employed to control the power supplied to the motor and to achieve commutation. Synchronization between the firing signal supplied to the controlled rectifier used as the power control device and the firing signal supplied to the controlled rectifier used to commutate the power device has been too difiicult to achieve in practice. When these firing signals are not properly synchronized, the power device may fail to commutate, and effective control of the power supplied to the load is lost.

Silicon controlled rectiers and their associated firing circuits are sensitive to the effect of circuit transients. Frequently, as a result of such circuit transients, the controlled rectifiers may be misfired or they may fail to commutate. As a consequence, power may be delivered to a loadat `a time when it is not needed or wanted. In

`applications where the motor is used to drive a vehicle, an

accidental firing of a silicon controlled rectifier may cause Vthe vehicle to move unexpectedly and cause injury to personnel. It is desirable, therefore, to provide a control circuit employing solid state controlled rectifiers wherein provision is made for positively insuring against misfirings or a failure of the controlled rectiners to commutate.

Accordingly, it is the general object ofthe present invention to provide an improved cont-rol circuit and .apparatus employing solid state power devices for supplying power from a direct current source, such as a battery, to a load.

It is another object of the invention to provide an improved control circuit employing silicon controlled rectiers for operating a motor whereby failures to commutate and misfirings of the solid state controlled rectifiers are effectively minimized.

A more specific object of the -present invention is to provide an improved control circuit employing silicon controlled rectiflers for selectively Varying the speed of a motor.

In accordance'with one form of my invention, I have provided an improved control circuit for selectively varying the average power supplied to a load from a potential source. A first solid state power device having a plurality of semi-conducting material zones of P and N types forming more than two P-N junctions, is fired at selected frequencies to control the power supplied to the load. The end zones of the first device are connected in series with the source and the load whereby current is supplied to the load when the intermediate junction of the device is forwardly biased and current to the load is blocked when the intermediate junction is biased in a reverse direction. A first firing circuit means is coupled with the first device to supply a firing pulse to the intermediate zone @at a selected frequency. The first firing circuit means is connected in circuit across the end zones of the first device so that it 4is operative in the circuit only when the first device is in a blocking state.

A second solid state power device similar in con-struction to the first device is included in the circuit and has one of its end zones connected in circuit with an end zone of the first device. A capacitor is provided having one side connected with one end zone of the first device and having the other side connected with one end Zone of the second device. The said other side of the capacitor is positive with respect to the one side when the first device conducts.

A second firing circuit means is connected in circuit with the first device whereby the second firing circuit means is operative only when the first device is in a conducting state. This second firing circuit means is coupled with the intermediate junction of the second device. When the intermediate junction of the second device is forwardly biased, the second device conducts and the capacitor is discharged to cause the first device to revert to a blocking state.

In a more specific form of my invention, I have provided an improved control circuit for selectively varying the average power supplied to a motor from a direct current source. A first solid state controlled rectifier is connected in a series circuit with the armature of the motor, and is fired at a selected frequency by a firing circuit whereby controlled pulses of power are supplied to operate the motor. A commutating capacitor is connected across the first controlled rectifier and is discharged by a second controlled rectifier at a predetermined interval after the first controlled rectifier is switched into a conducting state to commutate the first controlled rectifier. A second firing circuit is coupled with the second controlled rectifier and is connected in the conducting path of the first controlled rectifier so that it is operative only when the first c-ontrolled rectifier is in a conducting state. Also, to further insure the controlled rectifiers are properly switched on and olf, the first firing circuit is connected across the first controlled rectifier. fier is in a conducting state, the first firing circuit is in effect short-circuited and is inoperative. Thus, the first firing circuit is operative only when the first controlled rectifier is in a blocking state and the second firing circuit is operative only when the first controlled rectifier is in a conducting state thereby minimizing possible mislirings of the cont-rolled rectifiers and failures of the controlled rectifiers to commutate that might adversely affect the control of the motor speed.

When the first controlled recti- According to another aspect of my invention, I have provided an impedance element, such as a resistor, in series circuit with a free-wheeling diode connected in shunt with the motor armature. It was found that this impedance element had a dynamic braking action on a shunt motor when the motor was operated as a generator; For example, when the control circuit was used to operate a drive motor for an electric caddy cart, the impedance element in conjunction with free-wheeling diode providedA a braking action when the caddy cart moved in a reverse direction due to a steep slope tandcaused themotor to operate as a generator. v This braking action was sufficient to prevent the cart from suddenly` running away in a reverse direction and to allow the operator time to regain control of the drive motor.

The subject matter which l regard as my invention is set forth in the appended claim. The invention itself, however, together with further objects and advantages thereof may be understood by` referring to the following description taken in connection with the accompanying drawing which is a -schematic circuit diagram of an irnproved control circuit for a DC. motor embodying one form of my invention. Y Y

Referring now to the drawing in more detail, the control circuit or apparatus shown therein isy generally identified by the reference numeral lil `and is adapted for connection to a suitable direct current source, such as a battery 11 Ior a filtered rectified A.C. supply. The apparatus 1f) is connected in circuit with the direct current vsource 11 through a switch 12 and an input means comprised of terminal leads 13, 1li. 1 In the illustrated'exemplification of the invention, th control circuit or apparatus it) is used to control the speed ofa D.C. motor, which is shown schematically by the armature 15 anda serially connected field windinglo. lt will be understood that the control circuit l@ lmay be used to .selectively vary the average power supplied to other loads.

The speed of the motor is controlled by varying the frequency of the current pulses supplied to the armature 15 and field winding 16. vThe frequency of the current pulses is controlled by a silicon controlled Vrectifier SCR1 and its associated firing circuit 17 shown enclosed in the dashed rectangle. The duration of these pulses is controlled by another silicon controlled rectifier SCRZ, its associated firing circuit 1? shown enclosed in a dashed rectangle, and acommut-ating capacitor C1. Although in the illustrated exemplifiation of the invention, the duration lof the currentpulses is fixed, it will Vbe appreciated that the duration may be varied, if desirable, by varying the rate at which firing pulses are provided by firing cirf The commutating capacitor C1 and the controlled rectifier SCR2 are connected acrossthe controlled rectifier SCR1 so that the capacitor C1 is discharged when the controlled rectifier SCR1 is switched on. The capacitor C1 is charged through diode D1 with the current induced in the secondary of Vthe transformer T1. With this circuit arrange- `ment, it is possible to use a smaller commutating capacitor C1 since full load current is not handled by it. The commutating capacitor C1 has to store only sufficient energy required for the commutation of the controlled rectifier SCR1.

The general arrangement of a capacitor and a second controlled rectifier to effect commutation of a first controlled rectier is described at pages 112 to 119 of the erator.

publication entitled The General Electric Controlled Rectifier Manual (first edition, 1960) published by the Semiconductor Products Department, General Electric Comparo Syracuse, New York. But as will be'hereinalter more fully explained, the present invention provides an improvement in the control of the commutation whereby failure to commutate and misfiring of the controlled rectifiers are minimized.

As the armature current increases, it will be appreciated that the current handled bythe controlled rectifier SCR1 increases and that the amount of energy required to commutate the controlled rectifier SCR1 will also be increased. Ene to the transformer action of the transformer T1, more energy is stored in the commutating capacitor C1 as the current handled by the controlled rectier SCR1 increases. Consequently, energy stored in the commutating capacitor C1 is varied to meet the requirements of controlled rectier SCR1 as load conditions vary.

in order to provide a path for the inductive decay current of the armature l5 and field winding L6 when the controlled rectifier SCR1 is turned off, a free-wheeling diode D2 is connectedv in shunt with the armature l5 and field winding 1d. The free-wheeling diode D2 permits this current toflow in a closed loop.

An impedance element, a resistor R9, was connected in circuit with the free-wheeling diode D2 to provide a load for the armature l5 when it was driven as a gen- It was found that when the motor armature 1S was used to drive a caddy cart, the resistor R9 in conjunction witn the diode D2 provided a dynamic braking action to arrest the movement of the motor when it was driven by the wheels of the caddy cart. Thus, when'the 'caddy cart suddenly moved in a reverse direction due to its own momentum, as frequently occurs where the caddy cart is operated on steep slopes, the arrangement of the impedance element R9 and free-wheeling diode D2 arrested the backward movement of the vehicle to permit the operator to regain control of the vehicle with the motor.

The silicon controlled rectifiers SCR1 and SCR2 are essentially PNPN semiconductor devices. They are formed of a plurality of semiconductor zones of P and N type material forming P-N junctions. It will be noted that in the devices used in the exempliiication of the invention three junctions were employed. In the drawing, the anode of the silicon controlled rectifiers SCR1 and SCR2 is represented by the connection at the zone end Azone of P-type material, the cathode is represented by the connection at the end zone of N-type material, and the Vgate of the controlled rectifier is represented by the connection at an intermediate zone of the P-type material of the controlled rectifiers SCR1, SCR2.

Y It will be appreciated that the potential from the direct current source lll is normally insuicient to bias the controlled rectifiers SCR1, SCRZ into conduction, and a signal must be applied atthe gates of the silicon controlled rectifiers SCR1, SCR2 to cause the intermediate junction of the controlled rectifiers to be biased in a forward direction and switch the rectiers into a conducting state. To turn od controlled rectifiers SCR1 and SCR2 and thereby permit the gate of the controlled rectifier to regain control, it is necessary that theanode voltage of the controlled rectifier be reduced to zero or a reverse bias be applied across the end zones for a finite length of time. In the illus- Vwill flow to the commutating capacitor C1 through the secondary'winding S1 and diode D1. The voltage across the capacitor lC1 will be such that the lower end thereof, as seen in FIGURE 1 is positive with respect to the other end. Thus, when the controlled rectifier SCR2 is switched into conduction, the commutating capacitor C1 discharges to stop the conduction of the controlled rectifier SCR1.

A serially connected resistor R12 and diode D4 are connected in shunt with the secondary winding S1 to provide a path for the inductive decay current of transformer T1 as the load current is turned off by controlled rectifier SCR1. When the current in theprimary P1 is cut off, the inductive decay current results in a voltage across the secondary S1 that is opposite in polarity to the normally induced secondary voltage. Thus, diode D4 is forward biased and provides a shunt path for the decay current.

The output of firing circuit 17 is coupled with controlled rectifier SCR1 to provide pulses at the gate of controlled rectifier SCR1. The frequency of these pulses is determined by the setting of the variable resistor R1 which serves as the speed control means. The firing circuit 17 is essentially a relaxation oscillator with its input connected across the controlled rectifier SCR1 and includes a unijunction transistor Q1, resistors R2 and R3 connected with the base-one and base-two electrodes 19, 20, respectively of transistor Q1, a charging capacitor C2, diode D3 and a pulse transformer T2.

The Zener diode Z1 is connected across the firing circuit 17 in order to limit the voltage applied across the firing circuit to 'the reverse breakdown voltage of the Zener diode Z1. It will be appreciated that for voltages below the breakdown value, the Zener diode Z1 acts as a rectifier and when the reverse voltage exceeds the breakdown value the Zener diode Z1 presents a very low resistance. The Zener diode used in the exemplification of the invention had a reverse breakdown voltage of 27 volts and the battery 11 was rated at 36 volts. The voltage drop across the resistor R4 was used to make up the difference between the 27 volts across the Zener diode Z1 and the source voltage. Resistor R5 performs a similar function for firing circuit 18. Preferably, resistor R4, R5 are separate resistors since with a single resistor used in the connection between points B and F, it was found that a certain amount of feedback occurred between the firing circuits 17, 18.

The rate at which the charging capacitor C2 is charged to the peak emitter Voltage of the unijunction transistor Q1 determines the frequency at which the unijunction transistor Q1 is fired. When unijunction transistor Q1 is fired, the capacitor C2 is discharged and a pulse of current is induced in the secondary of the pulse transformer T2, which is applied by leads 21 and Z2 across the gate and cathode of the controlled rectifier SCR1. Thus, when unijunction transistor Q1 is fired, controlled rectifier SCR1 is triggered into conduction.

Resistor R2 controls the discharge rate of the capacitor C2 while resistor R3 compensates for temperature changes in transistor Q1. A diode D3 was used to protect the unijunction transistor Q2 against transient voltages which might damage the transistor Q2. A diode D5 was provided to prevent the inverse voltage from being applied at the gate.

Firing circuit 18 is also essentially a relaxation oscillator and is similar in its configuration to firing circuit 17. Firing circuit 13 includes a unijunction transistor Q2, resistor R5, resistors R7, R3, connected to base-one and base-two electrodes 2,3, 24, a charging capacitor C3 and a pulse transformer T3. When the voltage across the capacitor C3 reaches the peak point value of the unijunction transistor Q2, unijunction transistor Q2 is fired causing the capacitor C3 to discharge. Hence, a pulse is induced across the secondary of the pulse transformer T3 to fire controlled rectifier SCR2.

Leads 25 and Z6 couple the output of firing circuit 18 with the controlled rectifier SCR2 and apply pulses of current at the gate. Diode D3 prevents an inverse voltage from being applied across the gate and cathode. The Zener diode Z2 maintains the voltage across the firing circuit 18 at substantially the breakdown level of the Zener diode Z2.

The operation of the speed control circuit or apparatus 10 is initiated fby closing the switch 12. With switch 12 in the closed position, current will liow through the speed control variable resistor R1 to the charging capacitor C2. When the voltage across capacitor C2 reaches the peak point value of the unijunction transistor Q1, the resistance between the emitter and base-one electrode 19 falls off to a low value and causes the capactor C2 to discharge. The discharge current from capacitor C2 produces a pulse across the primary of pulse transformer T2. A positive firing pulse is supplied at thetgate of the silicon controlled rectifier SCR1, and controlled rectifier SCR1 is switched into a conducting state.

When controlled rectifier SCR1 is in a conducting state, it will be seen that the firing circuit 17 is in effect shorted out and firing circuit 18 is now energized. It will be noted that the primary P1, the serially connected armature 15 and field winding 16, and firing circuit 18 are in the conducting path of the controlied rectifier SCR1. Thus, when controlled rectifier SCR1 conducts, these cornponents of the circuit are energized. Current is now being supplied in a path from the DC. source 11 through input lead 13, the controlled rectifier SCR1, through the primary winding of the transformer T1, the armature 15, the field winding 16, the input terminal lead 14 and to the negative side of the battery 11. Also, the cornrnutating capacitor C1 is charged by the current induced in the secondary of the transformer T1 through diode D1. In the hereinafter to be described exemplification of the invention, the capacitor C1 was charged to a plus volts with respect to the positive side of the battery.

At the instant that the controlled rectifier SCR2 is fired, the timing cycle of the second firing circuit 18 begins. Controlled rectifier SCR2 is fired a predetermined interval after controlled rectifier SCR1 is turned on. This interval is determinedv by the RC constant of the serially connected resistor R6 and capacitor C3.

As was previously mentioned, if it is desired to vary the conduction period of the controlled rectifier SCR1, in addition to frequency at which itis fired, resistor R3 may be a variable resistor. When capacitor C3 reaches the peak point value of the unijunction transistor Q2, the resistance between the emitter and the base-one electrode 23 drops ofi` causing the capacitor C3 to discharge. As a result, a current pulse is generated in the secondary of the pulse transformer T3, and controlled rectifier SCR2 is switched into conducting state. When controlled rectier SCR2 is in a conducting state, it causes commutating capacitor C1 to be discharged. Thus, the cathode of the controlled rectifier SCR1 is rendered more positive than the anode and controlled rectifier SCR1 is reversely biased thereby turning it off.

When controlled rectfier SCR1 is turned off, firing circuit 17 is again energized and operative, and firing circuit 18 is in effect deenergized or rendered inoperative. Capacitor C2 of firing circuit 17 is again charged and the cycle repeats itself. The speed of the armature 15 is readily varied by changing the frequency of the current pulses supplied to the armature as determined by the control setting of the variable resistor R1.

The control circuit shown in the drawing was constructed to operate a DC. motor rated at 11/2 horsepower. The following circuit components used are given by way of a specific example of the invention and not by way of limitation thereof:

Battery 11 36 volts.

Controlled rectifier SCR1 General Electric CSOB silicon controlled rectifier.

Controlled rectifier SCR2 General Electric C35B silicon controlled rectifier.

Resistor R1 250,000 ohms. Resistor R2 l0 ohms, Resistor R3 3,900 ohms. Resistor R1 5,600 ohms.

Resistor R5 5,600 ohms.

The motor control circuit 10 employing the foregoing circuit components operated the motor with'unidirectional pulses of current at a frequency range between 29 and 400 cycles per second. The speed was varied and reliably maintained between zero and 2,80() revolutions per minute.

From the foregoing description or the operation of the speed control Vcircuit it?, it will be apparent that the tiring circuitvll which controls the controlled rectiier SCR2 is energized only during the interval that the con# trolled rectier SCRl is conducting. Further, during this interval, the tiring circuit 17 which controls the controlled rectier SCRI is essentially inoperative. Conversely, when the controlled ,rectifier SCR1 is in a blocking state, tiring circuit 17 is operative While tiring circuit i8 is not. With such an arrangement,l it was found that synchronization between the tiring times of the controlled rectilierr-SCRl, SCR2 is insured. Thus, the possibility of either controlled rectiiers SCR, or SCRgbeing misfired'is effectively minimized. An important advantage of such an arrangement is that precise and reliable control'is achieved over themotor or other load energized by the control'circuit. Because of this advantage, the invention was particularly usefulin controlling the speed of a DE. motor used to drive a vehilcle such as a caddy cart where reliable control is necessary, if not essential, for safe operation.

it will be understood that the particular vernbodinient of the invention described herein is intended as an illustrative example of the invention and that the invention isn ot necessarily limited to such an embodiment. Although the particular embodiment of the invention was applied to a speed control circuit for a motor, it will be apparent the control circuit is readily adaptable to Resistor R6 30,000 ohms. Resistor R7 3,900 ohms. Resistor R8' 10 ohms. Capacitor C1 l2() microtarads. y Capacitor `C2 .06 microfarad. Capacitor C3 .33 microfarad. Transformer T1 GE. 9T45Y7009 trans A former. Transformer T2 GB. 9T41Y1 pulse trans- Y former. Transformer T3 GE. 9T41Y1 pulse transformer. Diode D1 1N2l57. Diode D2 lN2l57. Zener diode Z1 27 volts, l Watt. Zener diode Z2 27 volts, l watt. VUnijfunction transistors Q1, Q?, 2N492.

other applications where it is desired to control the A speed control circuit for a motor having an armature and a iield winding, said motor being operated from a direct current source, said speed control circuit comprising: an input means adaptedfr connection to the direct current source, a iir'st controlled rectifier having an anode, a cathode and a gate, a transformer having a primary and a secondary, terminais adapted for connection to said armature and said eld winding to supply power thereto, circuit means connecting said primary of said transformer and said terminals in the conducting path of said rst controlled rectilier and with said input means, a r'st relaxation oscillator having an input connected across the rst controlled rectiiier and having its output connected in circuit With said gate of said rst controlled rectitier, said first relaxation oscillator including a unijunctio'n transistor having anernitter, a base-one and a base-two electrode',

a variable resistor and a charging capacitor connected in circuit with the emitter electrode of said unijunction transistor, a Zener diode connected across the base-one and base-two electrodes thereof, said variable resistor controlling the frequency at which signals are provided at the output or said first relaxation oscillator, acommuntating capacitor, a diode means, said commutating capacitor and said diode means being serially Vconnected in circuit with the secondary of said transformer, said diode means being poled to conduct current to said commutating capacitor when said rst controlled rectier isV in a conducting state, a second controlled rectiier having an anode, a cathode and a gate, said second controlled rectier being connected in circuit with said commutating capacitor and said rst controlled rectifier to cause said commutating capacitor to be discharged and stop conduction of said iirst controlled rectifier when saidvsecond controlled rectifier is turned on, a second relaxation oscillator having an input connected in the conducting path of said 'first controlled rectier and having its output coupled with the gate ot said second controlled rectifier, said second Vrelaxation oscillator including a unijunction transistor having a base-one, a base-two and an emitter electrode, a

yserially connected resistor and a charging capaictor connected in circuitwith the emitter electrode, and a Zener diode connected across the base-one and base-two electrodes, said second relaxation oscillator providing a signal at the gate or" said second controlled rectier a predetermined interval after said iirst controlled rectier is switched on and said second relaxation oscillator being operative only when said rst controlled rectier is in a conducting state.

References Cited in the file of this patent UNITED STATES PATENTS w 3,041,478 Gabor lune 26, i962 3,064,174 Dinger Nov. 13, 1962 3,193,616 Cole et al.V Sept. l0, 1963 OTHER REFERENCES Publicationf GE SCR Manual, First Edition, Auburn, New York, i960, pages 57, 58, i12-119, TK2798 (34g, 1960. 

